Direct current is applied to olive mill wastewater effluent which is held between two electrodes in a vertical
mode. Wastewater with a total dissolved solid and COD of about 39,300 and 120,000 mg/l, respectively, is used.
The effect of pH, voltage, current, and bed height on the dewatering process is investigated. The results, under
certain conditions, showed that EOD is an energy saving process for water removal and can be used for treatment
of olive mill wastewater. The process revealed high efficiency in reducing COD and total dissolved solid (TDS).
Water medium converted from acidic to basic upon application of direct current. Increasing the voltage or the current
enhanced the EOD process. Use of additives at certain level also enhanced the dewatering process; addition of alum
or electrolyte at certain concentration enhanced the removal rate. Sodium chloride is the most effective electrolyte
in EOD process. The on/off test, although saves energy, but did not perform better in the term of percentage water
removal compared to that of the continuous process, without on/off test.

Keywords

Olive mill wastewater; EOD; Direct current; COD; TDS

Introduction

Wastewater produced from olive oil processing constitutes a major
environmental impact and a problem which cannot be easily solved
by the specific agricultural industrial sector. This is due to the high
non-biodegradable organic content of such waste. The toxicity of this
type of wastewater is due to the high concentrations of polyphenolic
compounds. Typical olive mill wastewater is characterized by very
high chemical oxygen demand, COD (40–200 g/l), biological oxygen
demand, BOD (12–60 g/l), total solid content (40–150 g/l) and acidic
pH (about 5). The uncontrolled disposal of olive oil mill effluent
leads to intense phytotoxic phenomena in flora, quality degradation
of ground water reservoirs, surface aquatic reservoirs, and seashores
and sea. Furthermore, evaporation from the wastewater lagoons may
lead to unpleasant odors and increase of the insect population in those
areas, which is inconvenient to the nearby living inhabitants.

Various treatment technologies are available for treatment of
OMW. For example, Hayek et al. [1] used the up flow anaerobic sludge
blanket (UASB) reactor for the treatment of the olive mill wastewater.
They used two different laboratory scale reactors of 3-liter and 15-liter
size. They found that COD reduction efficiency reached 75% in both
cases and the biogas production was 79.6% of methane. Boari et al. [2]
used a lysimeter in a pilot scale to simulate a cell of a sanitary landfill.
They claimed that the landfill could act as anaerobic filter and reduced
the pollution load of the OMW while acting as a temporary storage
tank. Their results showed that the COD reduction efficiency was
70%. Electro-osmotic dewatering is an energy saving water removal
process and can be used for treatment of olive mill wastewater. Electroosmosis
dewatering (EOD) is a technique that removes water by
placing a colloidal material between two electrodes. This is based on the
electrostatic effects of the electrochemical double layer that is formed
at the particle water interface of the colloidal material. The polarization
of water molecules near the solid-liquid interface causes an electrical
double layer to form [3]. In this double layer, the charges on the surface
of the particles are electrically balanced by the opposite charges in
water, when an electric field is applied across the medium, the electric
double layer causes the motion of the particles (electrophoresis) and
the liquid (electro-osmosis); electrochemical reactions at the electrodes
occurred based on the electrolysis of water in the bed. Electro-osmosis
dewatering has been studied for its proven advantages [4] in term of high efficiency, low energy consumption and being free from blockage
of the filter media by sludge particles at high concentration as occurred
in filtration (mechanical dewatering). It has been proven that the energy
saving by EOD is about 75% compared with vaporization of water [4].
The technique has been used for concentration of food materials [5-8],
activated sludge [9-11], contaminated sediment [12] and organic soil
[13].

Effects of different parameters on the EOD process have been
significantly considered in literature. For example, Lockhart [14] has
studies the effects of different factors on the electro-osmosis dewatering
of sodium kaolinite suspension. The author started with a constant
voltage and then increased the input voltage to improve the process.
His study showed that the addition of NaCl salt or HCl acid decrease
the voltage needed to remove higher percent of water at the same
voltage. Also he concluded that there was no relation between water
flux per unit charge, from one side, and ion concentration and zeta
potential from other side. Ju et al. [15] have studied the electroosmosis
dewatering of bentonite under constant voltage and DC (Direct
Current). They determined the effects of initial solid content, initial
bed height, constant applied voltage, constant applied current and the
concentration of added CaCl2 on the EOD of bentonite. They found
that 60% of water can be removed by electro-osmosis, and the required
energy was less than that required to the vaporization water. It was
concluded that as the bed height decreases the energy of dewatering
decreases and the water flux per unit charge increases. Rabi [16] has
investigated EOD of bentonite suspensions under continuous and
interrupted DC voltage and current. Yoshida et al. [17] have examined
the electro-osmosis dewatering of activated sludge under AC electric
field with periodic reversal of electrode polarity. The authors found that the application of AC (alternate current) electric field resulted in a
decrease of electrical resistance and that the final amount of removed
water was higher than that under DC electric field. Chen et al. [18]
has dewatered vegetable waste electro-osmotically starting with high
moisture content. They found that there is proportionality between bed
height and percent of water removal and that as pH goes to neutral
value the percent of water removal increases and vice versa. Zhou et
al. [19] has worked on a new method to remove water from a sludge
using a horizontal electric field in order to facilitate the runway gases
produced at the electrodes and to keep the anode immersed in water
during the dewatering process. The authors compared their method
to those operated in a vertical electric field, and concluded that the
method described in their study has high efficiency, simple structure
and ease of operation. An attempt has also been made by Tezcan et
al. [20] to remove COD using poly aluminum chloride, as coagulant aid,
by an electrochemical method using either iron or aluminum as
sacrificial electrode. The apparatus constructed by those authors was
very simple one and the parameters considered were limited. They have
employed cylindrical container in which the suspension is added and
continually mixed with a stirrer; while one pole of the power supply
was connecting to the reactor operating as a cathode, the other was
connected to a stirrer as an anode. Recently Iwata et al. [21] studied
enhancement of dewatering process by analysis of combined electroosmosis
dewatering and mechanical expression operation. Another
recent investigation by Hsu et al. [22] considered the unified analysis
of dewatering and drying of sludge cake. The constant voltage and
constant current electro-osmosis, combined with vacuum filtration
were adopted to dewater the fine coal [23]. Curvers et al. [24] studied
the influence of ionic strength and osmotic pressure on the dewatering
behavior of sewage sludge; they found that an increase in the bulk ionic
strength brings about an increase in the final solid volume fraction
upon constant pressure filtration or centrifugation. An advanced
study was considered by Yang et al. [25] who investigated the structure
evolution of wastewater sludge during electro-osmosis dewatering by
means of Pore size distribution (PSD), box-counting fractal dimension,
and volume change of sludge matrix.

Although free disposal of OMW into the environment is not
permitted by law, the wastewater from different olive mills located in
different areas of Jordan is being disposed into the valleys, affecting the
soil, groundwater and water courses downstream. Therefore, OMW
must be treated before being discharged into receiving water bodies
or being used for irrigation purposes. In this work, the possibility
of treating OMW by electro-osmosis dewatering (EOD) process is
considered using bench-scale experimental apparatus designed for
such purpose. Specific objectives include study of the effect of voltage,
current, and bed height on the dewatering process; explore the
effectiveness of the EOD by estimating water flux per unit charge and
energy of dewatering per mole of water removed; study the effect of
EOD on the degradation of chemical oxygen demand (COD) of the
olive mill wastewater; and investigate the effect of adding coagulant
and electrolyte on the EOD performance. The effect of ionic strength
and intermittent operation to save energy are also considered.

Materials and Methods

Experimental apparatus

Electro-osmosis dewatering apparatus has been constructed in this
work. A schematic diagram of the apparatus is shown in Figure 1. This
is a bit different than the set up employed by Tezcan et al. [20]. The
apparatus consists of a cylinder fitted with two electrodes. The cylinder
is used to contain the sample of wastewater. The two electrodes are disks made of stainless steel; the diameter of the disk was 8 cm. Each
disk contains 174 holes arranged in diamond manner; the diameter
of the hole was 3 mm. Wires are fixed to both of the electrodes using
epoxy glue and are connected to a D.C. power supply. There are holes
drilled on the bottom electrode to provide drainage of both water and
gases produced by electrolysis. A filter cloth is placed on top of the
disk to prevent the colloidal material from clogging the holes. A plate
is placed underneath the lower electrode to collect the water drained
from the sample. This plate is connected to a gradual cylinder to
measure the volume of the water removed during the experiment. The
other electrode is connected to a piston, which has a mass of 556 g, and
placed on the top of cylinder. When pressure is applied, which is just
the weight of the piston and the atmosphere (i.e. 102.41 kPa), the upper
electrode presses the tailing bed in the cylinder and closes the electric
circuit for electroosmosis. The electrodes are chosen to be anode (+ve)
on the top and cathode (-ve) on the bottom, due to zeta potential [26].
The experiments have been carried out at room temperature. Detailed
description of the apparatus and experimental procedure are provided
elsewhere [6,7].

Figure 1: Electroosmosis dewatering apparatus.

Materials and analysis

A sample of olive oil mill wastewater was obtained from certain
olive oil processing plant in the northern area of Jordan. Olive mill
effluent wastewater was collected during the olive harvesting seasons
(October-January 2008). The sample of wastewater was collected
in metal container, which was kept closed and preserved at low
temperature.

The following variables were measured experimentally using
certain analytical procedure: pH, total solid content (TS), and chemical
oxygen demand COD.

pH: A digital calibrated pH meter made by WPA (Wissenschaftlich
Technische Werkstatten, Germany) was used to measure the pH of the
olive mill effluent wastewater samples.

TDS and COD: TDS and COD were measured following the
APHA methods [27].

Each experiment was carried out in duplicate and the average
results are presented in this work if they did not differ by more than
7%. Blank experiment was conducted without applying electromotive
forces; it was noticed that almost no water passed through the electrodes
without applied voltage.

Effect of various parameters on EOD operation

Effect of voltage: Tests have been conducted to study the effect of
voltage on the electro-osmosis of OMW. At certain voltage (10 or 15
or 20 V), a suspension of 200 ml was prepared and maintained at fixed
height of the bed (3.1 cm).

Effect of current: The effect of current variation on the EOD of
OMW has been investigated. In this case, the bed height was maintained
at 3 cm and EOD was conducted using three different currents, namely
0.1, 0.25 and 0.5 A and EOD were followed.

Effect of bed height on the EOD of OMW: Initial bed height is one
of the most important variables in EOD technology. Its effect on OMW
treatment is studies using two bed heights, namely 2.0 and 4.0 cm. The
initial solid concentration and electrical voltage were fixed at 4.4 wt%
and 20 V, respectively.

Results and Discussion

Effect of voltage on EOD of OMW

The results are shown in Figure 2A, B and C for rate of water
removal, current variation and energy of dewatering, respectively, at
each of the above-mentioned applied voltage. The rate of water removal
was almost constant during the first stage of the process, and started
increasing after 30 min. These results also showed that an increase
in applied voltage resulted in an increase in the dewatering rate. It
should be noted that the effective force on the dewatering process was
the electrical potential only; neither external pressure nor bed height
were varied during the EOD process of this work. This effective force
is governed basically by Ohm’s law (V = IR). The potential direction of
the power was constant form the upper electrode (anode) to the lower
one (cathode), thus the solid particles were attracted to the anode while
the water decreased at the portion closer to the cathode where the
water content is high and higher driving force for the water molecules.
The dewatering rate ceased when the electrical resistance attained
its maximum value and the effect of electrical field was minimum. A
similar trend was reported in literature for EOD using other types of
suspension [28].

Figure 2:Rate of water removal (A), current variation (B) and energy of
dewatering (C) for during dewatering of OMW at different applied voltages
and bed height of 3.1 cm.

It is also seen (Figure 2B) that the current is decreasing, indicating
an increase in the resistance while dewatering process taking place.
The instantaneous energy of dewatering is expected to decrease with
percentage removal (Figure 2C), which is due to the decrease in the
electrical current, since E = I2R. This is achieved at a voltage of 20 V, but
this is not the case at the beginning of the dewatering process for the
applied voltages of 10 and 15 V. The removal rate at the start up of these
experiments is maintained zero, thus the process need a lot of energy
to start dewatering process; when water is collected, the energy of
dewatering decreased with time, although the accumulative energy of
dewatering is increased with time. A similar conclusion was obtained
for electro-osmotic dewatering of tofu residue (okara) [5].

Characteristics of the wastewater before and after EOD treatment
are summarized in Table 1. The original wastewater is acidic with high
content of COD which is due to highly organic matters including nitrogen compounds, sugars, organic acids and phenols which increase
their organic loads. The TDS is also very high (39300 mg/L) in the
original solution, but is significantly reduced upon EOD treatment.
The pH increases to an about 10.4, this could be due to the degradation
of phenolic compounds upon EOD treatment. It is seen (Table 1) that
COD and TDS have been reduced to 56.6% and 76.5 %, respectively,
by this filtration technique. The high reduction in TDS of the solution
indicates that the solid material is retained on the electrodes through
the process.

The collected data are presented in the Figure 3 and Table 2. In general, the percentage removal increases with time [8,29-31]. It is
expected that as the current increases the percentage removal increases,
at any given period of time (Figure 3A). Following the dynamics of the
experiment, this is true only at the end of experiments. Figure 3B shows
that as the current increases the voltage increases for a given period of
time. This is due to the random generation of gas in the upper electrode which changes the resistivity of the bed and the degree of electrical
contact of the suspension and the upper electrode. According to these
results, the voltage increases with time until a maximum value (37 V) is
reached and remained constant. This is due to limitation of the power
supply used in the measurement, which can stand up to 37 V. Thus,
at this point the current began to decrease at the range of (0.2-0.09
A), although the value of current was fixed. Figure 3C shows that the
energy of dewatering decreases with time. This is for the same reason
explained above. The reduction in COD and TDS increases with the
increase in the applied current, i.e. about 73% the COD and TDS in
original sample has been reduced when the current was 0.5 A (Table
2). Al-Asheh et al. [8] studied the effect of current during electroosmosis
dewatering of tomato paste suspension. They also found that
the increase in applied current resulted in an increase in the rate of
dewatering.

As expected, the rate of water removal is lower at higher bed height
than that at the higher one (Figure 4A). This could be due to the low
electrical resistance at low levels of the bed, which causes a greater
effect of the electrical field. The amount of suspension in a 4 cm bed
height is greater than that in a 2 cm bed height. Thus, although the
percentage of water removed at higher bed height was greater than
that at lower bed height. Looking at the current variation, it is seen
(Figure 4B) that the current decreases sharper when using 2-cm bed
height than that of 4-cm bed height, which due to the same reason.
Variation in energy of dewatering (Figure 4C) are only occurred only at
low water removal for the two bed height; while it is the same for water
removal greater than 4%. Results for the COD, TDS and pH at the two
bed height are shown in Table 3. These measurement were taken for the
finally collected samples. It is seen that reduction in COD and TDS is
higher at 2-cm bed height than that at 4-cm bed height, while the pH
is almost constant.

Table 3: Characteristics of water after EOD process under constant voltage and
various bed heights.

On/off test

This test was conducted in order to study the possibility of saving
energy by turning the apparatus on for certain period of time and then
turning it off for another period of time. This was performed over a
period of 6 hrs. The results are presented in Figure 5 and Table 4. The
percentage removal of water is increasing with time, however, the case
of continues power supply, without on/off test, performs better than
the two scenarios of the on/off tests shown in Figure 5. This is due to
the effect of power on the dewatering process; since as power increases
the removal rate increases, and thus the initial amount of suspension
in an on and off process is greater than that in the continuous process.
This is also supported by the results displayed in Table 4. In general,
the current was decreasing with time, and also was decreasing with
the increase in the power consumption. The instantaneous energy of
dewatering was found to bethe lowest when using continuous power
supply; but largest value of the accumulative energy of dewatering was
noticed in the cases of 20 on and 10 off and 10 on and 20 off. This is
due to repeating the start up process in the experiment, as the start up
requires significant amount of energy.

Figure 5:Percentage of water removal for OMW at different power
consumption under constant DC voltage and different scenarios. Initial solid
concent. 4.4 wt %, voltage 20 V and bed height, 4 cm.

Table 4: Characteristics of water after EOD process under constant voltage and
various bed heights.

Effect of addition of coagulant

Electrocoagulation of olive mill wastewaters has been previously
investigated by other workers [20]. They found that the removal
efficiency of COD was increasing with the addition of coagulant and
that Fe was more effective than Al as an electrode. In this work, effect
of coagulant has been studied in order to see if it is possible to enhance
the dewatering process by this treatment. In this series of experiments a
solution of alum prepared in a concentration of 0.3 wt % and was added to the suspension before the beginning of the experiment with different.
Then EOD was conducted for about 5 hours. It was expected that the
percentage removal would increase with increasing the concentration
of the alum, since alume can agglomerate the particles together and this
in turns improve the dewatering process. This is clearly demonstrated
in Figure 6 after 200 min of the experiment. It was also noticed (data not
shown) that the increase in alum concentration decreases the energy of
dewatering. This would signify the importance of adding coagulant to
such this process.

The ionic strength, I, of a solution is a function of the concentration
of all ions present in a solution. It can be calculated by the expression:

where cB is the molar concentration of ion B (mol dm-3), zB is the charge
number of that ion, and the sum is taken over all ions in the solution.
Increasing the concentration or valence of the counter ions compresses
the double layer and increases the electrical potential gradient. The
use of electrolyte has also been identified as a means of improving the
kinetics and power consumption of electro-osmosis dewatering [24].
Sludge with high conductivity may reduce the rate of electro-osmosis
dewatering; hence, there is a limit for electrolyte’s concentration to be
effective. Three types of electrolytes have been tested in this, namely
CaCl2, NaCl and HCl. In this case, solutions of different electrolytes at
certain concentrations were prepared and been added to the suspension,
before the beginning of the experiment, at different amounts to result
in different concentrations in the final suspensions. Then EOD was
then conducted for about 5 hours.

Constant voltage tests were implemented with bed height fixed at
4 cm. The results for EOD of OMW using different concentrations of
CaCl2, NaCl and HCl are shown in Figure 7A, B and C, respectively. It
was expected that the percentage removal would increase with increasing
the concentration of the electrolyte; this is due to the increase in the
ionic strength of the solution which in turns improves the dewatering
process. However, the results did not show such trend for all electrolyte
concentrations. This could be due to that high concentration of the
electrolyte that results in a reverse effect on the dewatering process.
In the case of CaCl2, the low concentrations lead to negative effect on
EOD and an optimum concentration of 0.11-wt% CaCl2 resulted in
highest amount of water removal (Table 5). In the case of NaCl, the
optimum concentration for enhancement of EOD was 0.12-wt%; NaCl
concentrations higher or lower than this value did not effect the water
removal significantly. Low concentration of HCl depress the EOD of
ME, 0.035-wt%, while higher concentrations, 0.07 and 0.14-wt%, have
almost same behavior as that without electrolyte (Figure 7C). In general,
for all types of electrolytes, it was noticed that the current decreases
with time and that energy of dewatering decreases at the start-up of the
experiments and then reaches its smallest values when percent of water
removal was 5%. It can be concluded that the most effective electrolyte
on this EOD process was 0.12-wt% NaCl which resulted in the highest
percentage of water removal, and lowest amount energy of dewatering.
One drawback of using such chloride electrolytes is the corrosion of the
electrodes and hence electro-coagulation rather than electro-osmotic
dewatering. However, the concentrations of the electrolytes used in this
work were very low that electro-coagulation was not noticed during the
course of the experiments.

Figure 7:Percentage of water removed for OMW using different
concentrations of CaCl2 (A), NaCl (B) and HCl (C) under constant DC voltage,
initial solid content 4.4 wt %, and a voltage of 20 V.

Table 5: Characteristics of water after EOD process under constant voltage and
different CaCl2 concentrations.

Conclusion

Olive mill effluent wastewater is characterized by acidity (pH 4.52), high organic content (120000 mg COD/l), and large amount of total
solid (4.4-wt% solid content). It was found that the dewatering rate
increases with the increase in the applied voltage. At constant applied
voltage, the removal rate increases with time whereas current decreases
with time. Instantaneous energy of dewatering decreases with time.
At constant applied current, the removal rate increases with time
whereas voltage increases with time. The on/off test, although saves
energy, but did not perform better in the term of percentage water
removal compared to that of the continuous process, without on/off
test. Increasing the initial bed height decreases the dewatering rate, the percentage of removal water and energy of dewatering. Addition
of alum or electrolyte at certain concentration enhanced the removal
rate. Sodium chloride is the most effective electrolyte in EOD process.
The effluent of the EOD process still contains large degree of COD and
further treatment of such effluent using other wastewater treatment
techniques is needed.

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